The present invention relates to a rubber composition for a tread and a pneumatic tire, particularly a rubber composition containing modified epoxidized natural rubber.
In recent years, concern is rising regarding increase in oil prices and exhaustion of oil due to problems of supply. Also, from the viewpoint of environmental problems such as tightening regulations regarding resource conservation and prevention of carbon dioxide exhaust, natural resources are being reconsidered. The tire industry is not an exception and as an alternative material of synthetic rubber, natural rubber is attracting attention again. Natural rubber has high mechanical strength and excellent abrasion resistance and therefore is frequently used in large tires such as truck/bus tires. However, because natural rubber only has methyl groups with small molecular weight in the side chains and has a low glass transition temperature (Tg) of −60° C., there is the problem that gripping performance is poor.
In order to solve such problems, the method of using epoxidized natural rubber is suggested (see JP-A-6-220254, JP-A-7-90123, JP-A-7-149955 and JP-A-2001-233995). Epoxidized natural rubber is natural rubber wherein the unsaturated double bonds are epoxidized and molecular aggregating ability of the rubber increases due to the epoxy groups, which are polar groups. Consequently, epoxidized natural rubber has higher glass transition temperature (Tg) than natural rubber and is excellent in mechanical strength, abrasion resistance and gas permeation resistance. Particularly, in rubber containing silica, the silanol groups of the silica surface and the epoxy groups react and as a result, mechanical strength and abrasion resistance equivalent to those of rubber containing carbon black can be obtained. However, although epoxidized natural rubber has large hysteresis loss and is excellent in wet grip performance, there is the problem that rolling resistance is insufficient.
The present invention aims to provide a rubber composition for a tread, in which both improvement in wet grip performance and decrease in heat generating properties can be achieved while maintaining the excellent properties of epoxidized natural rubber such as mechanical properties and abrasion resistance, and a pneumatic tire using the same.